1. Field of the Invention
The present invention relates to a xerographic printing machine, and more particularly, to an improved transfer unit for a xerographic printing machine.
2. Description of the Related Art
In conventional xerographic printing machines, a transfer system, based on corona discharge, has been used for transferring a toner image formed on a photoreceptor to a print medium. This type of transfer system is disadvantageous in that under conditions of high humidity, the image transfer performance is poor. This is so because dust and other debris accumulate on the corona wire or corrugation of the print media occurs. To overcome this, it has been proposed to utilize a transfer system of the corona discharge type, in which the print media is pressed against a photoreceptor by a material having a volume electrical resistance of 10.sup.9 to 10.sup.14 .OMEGA. cm. However, this type of system can be disadvantageous because often the print media cannot be easily peeled from the photoreceptor. This will be described with reference to FIGS. 3, 4, and 5.
In FIG. 3, endless belt 1 is stretched, at a preset tensile force, around drive roller 2 and follower rollers 3 and 5, so as to press paper 15 against photoreceptor 8. FIG. 4 illustrates how a stack of papers 15, used as print medium, is cut during processing. When normal thin papers 15 are cut, several sheets of papers are stacked and cut. Accordingly, burr 16 may be created at the edge of stacked papers 15 after cutting, as shown in FIG. 4. The height of burr 16 often reaches several tens of .mu.m or more. When paper 15, having burr 16, is transferred in a state such that burr 16 of the paper is curved toward belt 1, paper 15 fails to peel off photoreceptor 8.
FIG. 5(a) is a graph showing the Paschen curve (indicated by a solid line) under atmospheric pressure, and typical gap-voltage vs. gap width curves (indicated by dotted lines) between two dielectric sheets with fixed quantities of charge. The graph is referred to in "ELECTROPHOTOGRAPHY", written by R. M. Shaffert and will be described below.
In the contact part (referred to as a nip part) of photoreceptor 8 and endless belt 1, paper 15 with burr 16 lies as shown in FIG. 5(b). As shown, gap 17 is formed between endless belt 1 and paper 15 by virtue of the presence of burr 16. As shown in FIG. 5(a), the potential difference Va+Vc between endless belt 1 and photoreceptor 8, and acting across gap 17, increases with an increase in the width of gap 17. Va is the voltage applied to endless belt 1, and Vc is the voltage on photoreceptor 8. These voltages are opposite in polarity so as to cause paper 15 to adhere to endless belt 1. When the gap voltage exceeds the Paschen curve, discharge occurs in gap 17, thus charging the leading edge of paper 15 with a positive charge. Consequently, paper 15 and photoreceptor 8 electrostatically attract each other and, when passing through the nip part, the paper is separated from endless belt 1, while sticking to photoreceptor 8.
After the leading edge of paper 15 sticks to photoreceptor 8, a minute gap is continuously formed between endless belt 1 and paper 15 at the releasing zone of the nip part, with rotation of photoreceptor 8 and endless belt 1. Accordingly, discharge continues in the gap between endless belt 1 and paper 15, so as to positively charge all of paper 15. This results in difficulty in separating paper 15 from photoreceptor 8.
When paper 15 is fed to the nip part along the surface of photoreceptor 8, even if the paper has no burr 16 at the edge or is located on photoreceptor 8 so that burr 16 is outcurved toward photoreceptor 8, a minute gap is continuously formed between belt 1 and paper 15 at a location immediately before the nip part. Accordingly, discharge occurs in the minute gap, and the above-mentioned paper peel-off failure occurs. To prevent peel-off failure, a conventional technique reduces the diameter of photoreceptor 8 or additionally uses a claw to separate paper 15 from photoreceptor 8.